1. Field of the Invention
The present invention relates to a semiconductor laser device having a substrate and an active layer, and in particular, to a technique which improves the electrical conversion efficiency of a semiconductor laser device for an excitation light source, such as a solid state laser such as an Nd-doped YAG (Nd:YAG) laser or a Yb-doped YAG (Yb:YAG) laser, a Yb-doped fiber laser, an Er-doped fiber amplifier, etc., as well as the electrical conversion efficiency of a semiconductor laser device for use as a light source for optical communication.
2. Description of the Related Art
In the past, a semiconductor laser device is formed, for example, by epitaxially growing, on an n type GaAs substrate, an n-type clad layer, a guide layer, an active layer, another guide layer, a p-type clad layer, a p-type contact layer, etc., in a sequential manner (see, for example, a first nonpatent document).
At this time, a carrier concentration of the GaAs substrate is set to about 1×1018 cm−3.
In general, it is said that a semiconductor laser device has a markedly high electrical conversion efficiency as compared with other laser devices, but further efficiency improvements are required under the strong demand of energy saving in recent years.